In electrophotographic recording techniques a "latent charge image" is produced on a photoconductor. This latent charge image is developed by applying an electrostatically charged toner which is then transferred, for example to paper, textiles, foils or plastic, and is fixed by means, for example, of pressure, radiation, heat or the action of a solvent. Typical toners are one- or two-component powder toners (also called one- or two-component developers); furthermore, special toners are also employed, examples being magnetic or liquid toners and polymerization toners (L. B. Schein, "Electrophotography and Development Physics"; Springer Series in Electrophysics 14; Springer-Verlag, 2nd edition, 1992).
One measure of the quality of a toner is its specific charge q/m (charge per unit mass). In addition to the sign and level of the electrostatic charge, the rapid obtainment of the desired charge level and the constancy of this charge over a prolonged activation period, in particular, is a decisive quality criterion. Moreover, the insensitivity of the toner to climatic effects such as temperature and atmospheric humidity is another important suitability criterion.
Both positively and negatively chargeable toners are used in photocopiers, laser printers, LEDs (light-emitting diodes), LCS (liquid crystal shutter) printers or other digital printers based on electrophotography, depending on the type of process and type of equipment.
To obtain electrophotographic toners or developers with either a positive or negative charge it is common to add charge control agents. As the color-imparting component in color toners use is typically made of organic color pigments. Relative to dyes, color pigments have considerable advantages on account of their insolubility in the application medium, examples being improved thermal stability and light fastness.
On the basis of the principle of subtractive colour mixing it is possible, with the aid of the three primary colors yellow, cyan and magenta, to reproduce the entire spectrum of colors visible to the human eye. Exact color reproduction is only possible if the particular primary color satisfies the precisely defined color requirements. If this is not the case, some shades cannot be reproduced and the color contrast is inadequate.
In the case of full color toners the three toners yellow, cyan and magenta must not only meet the precisely defined color requirements but must also be matched exactly to one another in their triboelectric properties, since they are transferred one after another in the same device.
It is known that colorants may have a long-term effect in some cases on the triboelectric charging of toners (H.-T. Macholdt, A. Sieber, Dyes & Pigments 9 (1988), 119-127). Because of the different triboelectric effects of colorants and, as a result, their sometimes highly pronounced effect on toner chargeability it is not possible simply to add the colorants to a toner base formulation once prepared. Rather, it may be necessary to prepare a specific formulation for each colorant, with the nature and amount of the required charge control agent being tailored specifically. This approach is correspondingly laborious and in the case of color toners for process color is just another difficulty to add to those already described above.
Furthermore, it is important for practical use that the colorants possess high thermal stability and good dispersibility. Typical temperatures for incorporation of colorants in the toner resins are between 100 C and 200 C when using compounders or extruders. Accordingly, a thermal stability of 200 C, or even better 250 C, is a great advantage. It is also important that the thermal stability is maintained over a prolonged period (about 30 minutes) and in different binder systems. Typical toner binders are resins formed by addition polymerization, polyaddition and poly-condensation, such as styrene, styrene-acrylate, styrene-butadiene, acrylate, polyester, phenolic and epoxy resins, polysulfones and poly-urethanes, individually or in combination, which may also include further ingredients, such as charge control agents, waxes or flow assistants, or may have these ingredients added subsequently.
Magenta pigments for electrophotographic toners and developers are employed in numerous forms. Typically employed magenta pigments are C.I. Pigment Red 122 based on quinacridone compounds, C.I. Pigment Red 48, C.I. Pigment Red 57:1, C.I. Pigment Red 146 and C.I. Pigment Red 184 based on azo compounds and C.I. Pigment Violet 1 and C.I. Pigment Red 8 based on triarylcarbonium compounds.
The magenta colorant C.I. Pigment Red 57:1 is of particular interest since it corresponds to the standard magenta shade. An alternative to C.I. Pigment Red 57:1 on account of its color is C.I. Pigment Violet 1, although this has poorer fastness properties, especially light fastness properties, so that in practical use in toners it usually has to be blended with the very much more lighffast C.I. Pigment Red 122 (2,9-dimethyl-quinacridone).
Fundamentally there is a need for a magenta pigment possessing a very high degree of transparency, blueness, good dispersibility and an extremely stable inherent triboelectric effect.
The term stable inherent triboelectric effect is understood as meaning that the pigment reaches its maximum triboelectric effect on the toners after a very short activation time, and that this level remains stable over a very long activation period. Toners and developers which exhibit unstable q/m values, i.e. whose level of charge depends greatly on the activation period, are correspondingly difficult to control.
Transparency is of central importance since, in the case of full color copiers or in printing, the colors yellow, cyan and magenta are copied or printed over one another, the sequence of the colors depending on the device. Consequently, if an overlying color is not sufficiently transparent, the underlying color is unable to show through to a sufficient extent and the color reproduction is distorted. In the case of copying or printing on sheets for overhead projection use, transparency is even more important, since in this case a lack of transparency even in just one color makes the whole of the projected image appear gray.